Method of making a coded drum



Jan, 5, 1965 A. LEHMANN, JR., 'ETAL 3,163,923

METHOD OF MAKING A CODE-D DRUM Filed Dec. 31, 1959 2 Sheets-Sheet 1 [III/[11111IIIIII77IIIIIIII VIII/I [Ill/III ANTHONY LEA/MflAN/Q g3 INVE 0R.

2 Sheets-Sheet 2 Jan. 5, 1965 A. LEHMANN, JR., ETAL METHOD OF MAKING A coDED DRUM Filed Dec. 31, 1959 United States Patent Ofilice 3,153,?23 Patented Jan. 5, 1965 3,163,923 METHOD OF MAKING A CODED DRUM Anthony Lehmann, Jr., West Caldwell, and Frank Peer, 51a, Boonton, N..l., assignors to General Frecision, Inc., a corporation of Delaware Filed Dec. 31, 1959, Ser. No. 863,2? 4 Claims. (Cl. 29-45555) This invention generally relates to improvements in the production and manufacture of rotary drums employed in rotary analog to digital converters and the like in which the rotational angular position of the drum is read out in the form of digital electrical signals.

In the prior art it is known to employ rotatable drums or like commutators having a predetermined pattern or code of conducting and non-conducting areas over its surface which are successively brought into contact with fixed brushes as the drum is rotated. The code markings on the drum or member vary at each angular position from those at other positions in a predetermined fashion, such as in a binary code pattern, whereby as the drum is rotated the brushes in sequentially contacting the conducting and non-conducting areas, transmit impulses representing the rotational displacement of the drum or a mathematical function thereof.

In making such drums or discs it is a common practice to form the conducting and non-conducting code portions by inserting and embedding individual metal segments in spaced arrangement into the interior of a drum, said segments projecting through the outer surface of the drum which is made of suitable plastic material in a manner that may be generally likened to the making of a mosaic. Each of such segments consists of a thin strip of precious metal, all being of the same width that is little greater than the width of a brush, but of variable length covering a predetermined length of are along the outer circumference of the drum.

After inserting the segments, means are additionally provided inside the drum for electrically interconnecting and commonly energizing the segments thus enabling the brushes to transmit an electrical potential upon engaging a conducting segment. Finally the drum is finish machined to remove all excess material from its circumferential surface and provide a smooth outer surfaced drum with the individual spaced metal segments being flush with the non-conducting drum material located in between the segments.

However, this type of construction suffers from a number of disadvantages. Initially, drums made in this manner are quite expensive, involving considerable special tooling and labor costs for fabricating the individual metal segments and subassembling these segments on the drum. This cost is compounded by the fact that many different types of drums and code patterns are in use and the production quantities for any given type are not sufficiently great to justify a large investment in automatic machinery for this purpose. Secondly, drums made in this fashion are subject to error due to misalignment of the individual segments during subassembly, and the assembled drum is also heavier and provides a greater moment of inertia than is desired due to the fact that additional metal connectors must be added to the drum to electrically interconnect the segments.

According to the present invention, there is provided an improved drum construction of this type and an improved method of making the drum that eliminates much of the cost and error of the prior methods. Generally speaking, the improved method of construction eliminates the individual fabrication of the metal segments and subassembly thereof and provides instead a unitary one-piece shell subassembly containing all of the metal segments in the predetermined pattern of spacing desired. This unitary subassembly is then easily molded onto a supporting shaft with a plastic drum fitted thereto and the molded drum unit is then finish machine to remove the excess molding material and provide the finished product desired. Since the individual segments are already interconnected together in the shell construction, the process of the present invention completely eliminates the step of making the electrical interconnections therebetween as practiced in the prior art and thereby eliminates considerable cost, as well as reducing the weight and inertia of the finished coded drum. To obtain the subassembled shell of interconnected metal segments in the desired pattern, there is employed a thin Walled hollow cylinder of conducting material having an outer diameter corresponding in size to the finished drum. This cylinder is variously slotted to provide a predetermined pattern of openings therein corresponding in arrangement and spacing with the non-conducting areas to be provided on the drum. By this technique the arcuate outside surface of the cylinder is supplied with a plurality of conducting areas in a predetermined pattern and with spaced therebetween being provided by the slotted openings through the cylinder which spacings are later filled by the nonconducting filler in the molding step to supply the smooth flush outer surface desired.

It is accordingly a primary object of the invention to provide an analog to digital converter drum of lighter weight and lower inertia.

A further object is to provide such a drum having greater accuracy, while being less costly to manufacture.

A still further object is to provide a less expensive and less difficult method of making such drums.

Still another object is to provide such a drum having greater resolution and accuracy.

Other objects and many additional advantages will be more readily understood by those skilled in the art after a detailed consideration of the following specification taken with the accompanying drawings wherein:

FEGURE l is a top plan illustrating a preferred hollow cylindrical shell subassembly slotted according to the invention;

FIGURE 2 is a sectional view lengthwise of the drum illustrating the slotted shell molded onto the shaft;

FIGURE 3 is a view similar to FIGURE 2 and illustrating the finished drum construction; and

FIGURE 4 is a view similar to FTGURE 1 illustrating a drum constructed according to prior art methods.

Referring now to the drawings for a detailed consideration of one preferred drum construction and its process of manufacture, there is shown in FIGURES 1 to 3 the various stages in the construction of the drum. In FIG- URE 1, there is shown a hollow cylindrical shell 16 made of a thin-walled precious metal tube and having six parallel rows of slots 11 to 17 formed through the wall of the tube in a predetermined circumferential configuration corresponding to the non-conducting areas to be provided on the finished drum. The outer diameter of the hollow cylinder it? is selected to correspond with the desired outer diameter of the finished drum The width of all of the various slots 11 to 17 is substantially equal and the spacing between the parallel rows is al o substantially equal, since in the finished unit, a series of brushes (not shown) are fixedly supported across or transverse to the drum and adjacent its outer surface, with each brush being adapted to contact the drum along a different parallel circumferential row as the drum is rotated past the brushes. Consequently, with this arrangement, the individual brushes alternately make contact with the ccnducting portions of the cylinder 10 and 3 fail to make contact therewith when the plastic material projecting through a slotted area lies beneath the brush.

The angle subtended by the various slots about the circumference of the drum varies in accordance with a predetermined code pattern whereby at any given angular position of the drum, the various brushes are engaging either a conducting or plastic filled slotted portion to transmit the desired digital signals.

After the hollow cylinder has been machined or otherwise fabricated to provide the slotted areas as shown in FIGURE 1, the hollow cylinder 10 is coaxially supported over a hollow shaft 18 of much smaller diameter (FIGURE 2) by a mold or the like (not shown) and a plastic material 19 is added to the mold by either an injection molding process or other method of molding to completely embed the shell 10 and fill the space between the shell and the shaft, as well as filling the slotted areas in the shell 10 with the plastic material, as shown in FIG- URE 2. The mold is so formed as to enable an excess of plastic material 19 to cover the outside of the shell 10 through the slotted openings in the shell, as best shown in FIGURE 3. This finish machining operation provides a smooth outer surface about the finished drum, which smooth outer surface contains the conducting and nonconducting areas as desired. Consequently, the series of brushes (not shown) may be positioned against the drum surface and continuously maintained in this contacting position without disturbance as the drum is rotated about its central axis 20.

Since in the ultimate installation a plurality of cascaded drums of this type may be employed and interconnected by drive gearing or other transfer mechanism, the drum unit is preferably molded with an outstanding cylindrical extension 21 and 22 at either end thereof, as shown in FIGURE 2. These extensions 21 and 22 serve as supports over which spur gears, or other type of transfer members, such as 23, 24, and 25 may be journaled, as shown in FIGURE 3, to permit the drum to be rotated about its central axis 20, and to enable rotation of the drum to in turn impart movement to the next succeeding drum in a cascaded chain.

Recapitulating briefly the steps in constructing the drum, the arrangement of conducting and non-conducting segments formed in a predetermined code pattern are connected together physically and electrically. This construction, therefore, eliminates the prior art step of electrically interconnecting the individual metal segments thereby reducing the weight and inertia of the finished drum, as well as appreciably reducing the production cost,

In forming the slotted cylinder 10 according to a desired pattern, the slots may be initially end milled in the cylinder 10 to approximately the required circumferential length and angular spacing. Thereafter the ends'of the slots may be broached to the required length and angular relation, while the tubular cylinder is accurately supported on an adjustable fixture for the purpose of preserving the cylindrical shape and dimensions of the hollow cylinder during the breaching operations. Alternatively, the milled slots may be finished to required size and spacing by a fine milling or end milling operation, if desired. Still another method of accurately forming the slots in the cylinder 10 would be by a machine engraving technique wherein a master templet of the desired configuration would be initially'made and a pantograph type machine engraver would automatically follow the configuration of the templet permitting a jewelers lathe having a diamond drill fitted thereto to accurately trace out the desired pattern of slots in the hollow cylinder 10. Other methods of slotting the hollow cylinder, such as by etching techmques, are well known and will be readily suggested to those skilled in the art.

In molding the slotted cylinder 10 onto the shaft and filling the slots with a plastic non-conducting material, the mold itself may be a simplified hollow cylindrical mold made of metal or the like having an end wall formed with a cylindrical groove to receive and retain the slotted cylinder 10, and a central opening to receive the shaft or bushing 18. A suitable plastic material, such as an epoxy resin, may then be added to completely fill the mold and embed the cylinder 10 about the bushing 19. Various otherknown molding techniques may of course be employed as desired.

For purposes of comparison, there is shown in FIG- URE 4 a digitally coded drum for this purpose, constructed according to prior art processes. As shown, in this usual construction, the drum comprises a central bushing member 26 supporting an outer hardened plastic layer 27. The individually made metal segments 28 are separately embedded in the outer plastic layer 27, and hence require the addition of conducting members 29 also embedded withinthe outer layer 27 for purposes of electrically interconnecting the segments 28. For interconnecting the gears 30 to 32 or other transfer components onto the bushing 26, a hollow tubular joining pin 1 or rivet 33 is passedthrough aligned openings through initially made by forming a plurality of slotted openings through a thin-walled hollow tube of precious metal, such as gold. This tube is then supported coaxially over a tubular shaft of smaller diameter and a non-conducting plastic material is added to completely embed the cylinder and fasten it to the shaft. After the plastic material has hardened, the unit is finally finish machined to remove the excess plastic material from the outer surface thereof until the outer surface of the conducting cylinder is exposed flush with the plastic material, filling the voids or slots in the cylinder thereby to provide the conducting and non-conducting areas over the surface of thedrum as desired. Since the position and configuration of the conducting segments are preformed when the one-piece hollow cylinder is initially slotted, there can be no error in alignment of the segments on the drum relative to one another during assembly. This is to be particularly contrasted with prior art constructions, wherein the conducting segments are individually made and individually fastened to the drum. Additionally, according to the present invention all conducting segments are an integral part of the slotted shell 10, and consequently are commonly these members and the opposite ends of joining pin or rivet 33 are headed or upset outwardly to form frustoconical heads at the ends of each pin, as shown. The bushing 26 is also formedwith outstanding substantially cylindrical pilot sections, at either end thereof to centrally support the gears or other transfer members. It is believed evident from a comparison of FIGURE 3 and FIGURE 4, that the drum construction of the pres ent invention requires considerably less metal than that of the prior art device, making the unit lighter in weight and having less inertia and being less expensive to produce. It is also believed evident that the process, of making the unit is considerably'more simplified than is possible with this prior art technique.

What is claimed is; 1. A method of fabricating a rotary analog-to-digital converter drum, comprising:

providing a thin-walled hollow cylinder of electroconductive material; forming a plurality of circumferential slots in said cylinder precisely spaced, dimensioned, and arranged to define a predetermined digital ,code pattern, all of said slots having a circumferential dimension smaller than the circumference of the cylinder;

disposing the cylinder conxially about a shaft of smaller diameter than the interior of the cylinder; and

embedding the cylinder and shaft in a hardena'ole, di-

electric material, completely enveloping the exterior of the cylinder and filling the slots therein and the volume between the shaft and interior surface of the cylinder; and

removing excess dielectric material from the exterior surface of the cylinder so as to leave cylindrical surface regions of dielectric material in said slots flush with the outer surface of the cylinder.

2. The method defined in claim 1 wherein said slots are initially formed by milling to a circumferential dimension shorter than the desired final dimension and the cylinder is thereafter broached to achieve the final desired circumferential dimension.

3. The method defined in claim 1 wherein the circumferential slots in the hollow cylinder are formed by engraving.

4. The method defined in claim 1 wherein the circumferential slots in the cylinder are formed by etching.

Reterences Cited by the Examiner UNITED STATES PATENTS JOHN F. CAMPBELL, Primary Examiner. MAX L, LEVY, WHITMORE A. WILTZ, Examiners. 

1. A METHOD OF FABRICATING A ROTARY ANALOG-TO-DIGITAL CONVERTER DRUM, COMPRISING: PROVIDING A THIN-WALLED HOLLOW CYLINDER OF ELECTROCONDUCTIVE MATERIAL; FORMING A PLURALITY OF CIRCUMFERENTIAL SLOTS IN SAID CYLINDER PRECISELY SPACED, DIMENSIONED, AND ARRANGED TO DEFINE A PREDETERMINED DIGITAL CODE PATTEN, ALL OF SAID SLOTS HAVING A CIRCUMFERENTIAL DIMENSION SMALLER THAN THE CIRCUMFERENCE OF THE CYLINDER; DISPOSING THE CYLINDER COAXIALLY ABOUT A SHAFT OF SMALLER DIAMETER THAN THE INTERIOR OF THE CYLINDER; AND EMBEDDING THE CYLINDER AND SHAFT IN A HARDENABLE, DIELECTRIC MATERIAL, COMPLETELY ENVELOPING THE EXTERIOR OF THE CYLINDER AND FILLING THE SLOTS THEREIN AND THE 